Set sets a variable to a certain value, and takes the form set variable value. It can set global variables that have been defined at the top of the code, and local variables that were defined by let. It can also set variables belonging to the agent who calls set (including turtle, breed, or patch variables defined by turtles-own or patches-own); for example,

turtles-own [age] 
ask one-of turtles [ 
    set age 10 ]

would set the age of a turtle to 10. Turtles can also access and set the variable of the patch they are standing on.